SECT, in.] CONDENSATION OF STEAM. 141 



than level with the upper edge of the fire flues. At the upper end it should be 

 provided with a pipe U, to convey down the hot water without the danger of 

 scalding any one, and the upper part should terminate in a higher pipe V, to 

 convey away the steam. The action of this safety tube is, first to lower the water 

 in the boiler to bring the feeding pipe into action, if it be not so before, and then 

 to allow the escape of the steam. I have had two boilers done in this manner ; 

 and the effect of endeavouring to render the steam stronger than it ought to be 

 is completely counteracted, and the boiler restored again to its regular pressure 

 in a few minutes after the tube has discharged its column of water. The dis- 

 charge of a portion of hot water by the tube, and the admission of colder by the 

 feed pipe, tends to lower the steam ; but by the feed pipe alone this does not 

 happen, as the hot water then rises in the feed pipe and prevents the entrance of 

 cold. Another advantage of this construction is, that should the water fall below 

 the mouth of the tube, the steam would escape, and if the noise of its escape did 

 not warn the engineer of the state of the boiler, the want of steam would soon be 

 a motive to look after it. 



The height of the tube for different pressures is easily calculated, for the height 

 of a column of water equivalent to 1 Ib. on the circular inch is, for ordinary 

 temperatures, 3'1 feet; hence for a pressure of 4 Ibs. on the circular inch, 



4 x 3-1 = 12-4 feet, the height for the tube, that is, equal to a little more than 



5 Ibs. per square inch. It is obvious that it is adapted only for low pressure 

 steam. 



265. Other modes have been proposed for constructing valves, some of which 

 are deficient in principle, others are complex in construction and operation. The 

 solid piston valve proposed by Chevalier Edelcrautz would either stick fast with 

 high pressure steam or allow a constant escape ; and to this difficulty is added the 

 nicety of fitting a solid piston so as to be and remain steam-tight. If an elastic 

 metallic piston be used instead of a solid one, the expense of construction becomes 

 considerable ; and a common packed piston is not to be depended on for the pur- 

 pose, its friction is so irregular. 



266. For high pressure boilers, more careful attention to the means of security 

 are necessary than for low pressure ones, as it is easy to show that the risk is much 

 greater; indeed some most dreadful accidents have had the effect of rendering 

 people more cautious respecting them. Several methods have been used to guard 

 against these accidents by Trevithick, who first brought the high pressure engine 

 into use. He proposed that the safety valve should be inclosed in an iron case 

 and locked, so that no person could get access to it to increase the load beyond 

 what was intended to be employed : he also had a hole drilled in the boiler, 

 which he plugged up with lead, at such a height from the bottom that the boiler 



